A turbine fuel refers to a fuel composition which may be burned in a turbine to provide power for various human activities. Turbines may be stationary, such as those used to generate electricity, or they may be used to power mobile platforms, such as providing power for ships. Turbine fuels meeting certain specifications may be used as jet fuel for airplanes.
Specifications for turbine fuel intended for use in jet engines are more stringent than those for fuels intended for use in turbines used to produce electricity. In particular, various grades of jet fuel have specifications which place limits on the freeze point. For example, Jet A has a freeze point of −40° C. or lower, Jet A1 has a freeze point of −47° C. or lower, and Jet B has a maximum freeze point of −50° C. or lower. While the specifications for a turbine fuel intended for use in generating electricity may differ from turbine fuel intended for use as jet fuel, it is more convenient to operate a refinery to produce only a single grade of product. Therefore, the most stringent specifications applicable for the intended use are relevant and must be met by the fuel. Where the specification density of conventional jet fuel needs to be maintained, these compositions of the invention are best used as blend stocks.
The Fischer-Tropsch process provides a way to convert a variety of hydrocarbonaceous resources into products usually provided by petroleum. These include jet fuel. In preparing hydrocarbons via the Fischer-Tropsch process, a hydrocarbonaceous resource, such as, for example, natural gas, coal, refinery fuel gas, tar sands, bitumen, crude oil, and fractions from crude oil, is first converted into synthesis gas which is a mixture comprising carbon monoxide and hydrogen. The syngas, in turn, is converted into synthetic hydrocarbonaceous compounds that have a predominantly linear structure, primarily n-paraffins, 1-alcohols, 1-olefins, and traces of other species. These hydrocarbonaceous species may be refined into various products, including turbine fuels.
European Patent Application No. 1015530, U.S. Patent Publication No. 2002/0005009A1 and U.S. Pat. Nos. 6,669,743; 6,309,432; and 5,766,274 describe the preparation of a Fischer-Tropsch derived product containing alcohols which is said to be useful as a jet fuel. However, this material has a density less than jet fuel specification. The presence of alcohols are claimed to improve the lubricity of the fuel. Unfortunately, the compositions taught in these documents employing the range of alcohols specific would fail to meet the freeze point specifications for jet fuel, and, consequently, they would not be suitable as commercial jet fuel. Furthernmore, the “jet” fuels using the higher concentrations of alcohols would also fail other jet fuel specifications such as water separation. The present invention is directed to Fischer-Tropsch derived turbine fuel compositions which are able to meet the freeze point and water separation (MSEP) minimum of 85 specifications for jet, preferably, enhancing yields in the process.
Freeze point represents the temperature below which solid hydrocarbons may form in aviation turbine fuels. Freeze point is determined by ASTM D 2386 which measures the fuel temperature at which solid hydrocarbon crystals formed on cooling disappear when the temperature of the fuel begins to rise.
As used in this disclosure the phrase “Fischer-Tropsch derived” refers to a hydrocarbon stream in which a substantial portion, except for added hydrogen, is derived from a Fischer-Tropsch process regardless of subsequent processing steps. The feed for the creation of the “Fischer Tropsch derived” refers to products derived from any carbon source, for example natural gas, coal, refinery fuel gas, tar sands, oil shale, municipal waste, agricultural waste, forestry waste, wood, shale oil, bitumen, crude oil, and fractions from crude oil.
As used in this disclosure the word “comprises” or “comprising” is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase “consists essentially of” or “consisting essentially of” is intended to mean the exclusion of other elements of any essential significance to the composition. The phrase “consisting of” or “consists of” are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.